1. Field of the Invention
The present invention relates to a measurement method and a measurement apparatus which measure a shape of a target, and a recording medium recordable by a computer in which a program which makes the computer execute a measurement of a shape of a target is recorded.
2. Description of the Related Art
Generally, a plurality of lenses are used in a camera lens or a projection optical system of a semiconductor exposure apparatus. It is because an aberration cannot be removed by only one lens. For example, a camera lens around 10 years ago was constituted by around 7 lenses. A semiconductor exposure apparatus around 10 years ago used around 30 lenses. In order to reduce the number of lenses to lower the cost, and realize a low aberration and a small space of the lens group, an aspherical surface lens is necessary. In recent years, the aspherical surface lens is also mounted in the camera lens or an optical system of the semiconductor exposure apparatus, and reducing the cost, lowering the aberration, and narrowing the space of the lens group are proceeding.
While the aspherical surface lens has a variety of advantages, there is a problem that the required process and the measurement are difficult. Therefore, the process of the aspherical surface and the measurement of the shape of the aspherical surface are fields in which researches are actively performed in recent years.
The measurement of the aspherical surface shape can be categorized into two types of method. A first measurement method is a contact mode method which measures a shape by tracing a surface using what is called a probe. Although the contact mode method can be widely applied, it takes much time for the measurement because it measures points on the surface one by one using the probe. Furthermore, the surface may get scratched by the contact of the probe. A second method is a non-contact mode and a measurement of an aspherical surface using an interferometer is typical.
As an aspherical surface measurement using an interferometer, there is a method in which a target is scanned in an optical axis direction. The method obtains a plurality of interference patterns while the target is scanned in the optical axis direction and processes each of the interference patterns. Subsequently, a complete shape is calculated. As a typical method, for example, there are U.S. Pat. No. 6,781,700 and Japanese Patent Laid-Open No. 2004-45168.
U.S. Pat. No. 6,781,700 discloses a measurement method of an aspherical surface shape which has an axisymmetric setting value. When a target is scanned in an optical axis direction, an annular interference pattern appears only in a certain area. Thus, in U.S. Pat. No. 6,781,700, the aspherical surface shape is calculated from the interference pattern by mathematically associating the interference pattern with the shape of the target. However, in this method, a highly-controlled interferometer and an advanced interference pattern processing is needed.
Japanese Patent Laid-Open No. 2004-45168 also discloses a measurement method of an aspherical surface shape which has an axisymmetric setting value. In this method, a plurality of annular interference patterns which appear in scanning a target in an optical axis direction are obtained. After an offset component of each of the interference patterns is removed, the interference patterns are corrected by 2 πn (“n” is an integer) to join them. According to the technology disclosed in Japanese Patent Laid-Open No. 2004-45168, the shape can be calculated by an easy method. However, as described later in detail, it is an approximate solution. Therefore, when an amount of scanning is large, the assumed approximation does not hold true and the measurement accuracy is deteriorated.
Thus, when the aspherical surface shape is measured by the interferometer, an advanced processing or an approximate calculation was necessary.